Light-induced desorption of trivalent chromium from adsorbents: One step closer to sustainability

Article abstract of DOI:10.1039/c8cc08099b

The environmental sustainability of existing adsorbents in remediating chromium-contaminated water has long been plagued by the necessity of using chemical desorbents in regenerating the adsorbents. Here, we demonstrate that trivalent chromium can be repelled from surface-bound merocyanine upon visible-light irradiation, avoiding any chemical desorbent, and thus secondary contamination, to restore the adsorption capacity of the exhausted adsorbents.

Full text of DOI:10.1039/c8cc08099b

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Light-induced desorption of trivalent chromium
from adsorbents: one step closer to sustainability†
Cite this: Chem. Commun., 2018,
4, 12770
5
a
a
a
a
ab
Xiaoyu Guan, Sunxian Yan, Jinming Chang,
Gaofu Yang and Haojun Fan*
Received 10th October 2018,
Accepted 22nd October 2018
DOI: 10.1039/c8cc08099b
rsc.li/chemcomm
The environmental sustainability of existing adsorbents in remediating a long-ignored problem that compromises sustainability of adsorp-
chromium-contaminated water has long been plagued by the necessity tion techniques in remediating chromium-containing water.
of using chemical desorbents in regenerating the adsorbents. Here, we
Ideally, an environmentally sound adsorbent should exhibit
demonstrate that trivalent chromium can be repelled from surface- high efficiency to capture chromium species in aqueous solution,
bound merocyanine upon visible-light irradiation, avoiding any while being capable of desorbing the retained chromium on
chemical desorbent, and thus secondary contamination, to restore demand without mechanical or chemical disruption. Recent
the adsorption capacity of the exhausted adsorbents.
advances in stimuli-responsive materials provide a unique
opportunity to address this challenge. By virtue of high spatio-
Since the last decade, concerns over contamination of aquatic temporal resolution of light, photoswitchable compounds able
ecosystems from the worldwide use of chromium in anthropo- to reversibly undergo light-induced transitions between two or
1
,2
genic activities have been escalating. This stimulates interest more metastable states with different physicochemical proper-
in developing ‘end-of-pipe’ techniques that enable efficient ties have been the current focus of attention for a wide spectrum
9
,10
removal of chromium contaminants from water before it was of emerging applications.
Among others, spiropyrans are an
disposed of or delivered. One of the most promising alternatives is important class of photoswitchable compounds. Spiropyran
3
adsorption, the majority of which employs solid surfaces with features an indole and a benzopyran moiety, jointed together
3
immobilized organic ligands to capture soluble chromium at the spiro sp -hybridized carbon atom leading to a perpendi-
4,5
species. In recent years, there has been a surge of publications cular orientation of both the planar heterocyclic rings. Upon
on new adsorbents suitable for remediating chromium- exposure to visible light, the spiropyran exists in a charge-
6–8
contaminated water.
These adsorbents are highly efficient, neutral, colorless, and closed form (SP), which isomerizes to
readily separable, and sometimes chromium selective. In addi- the corresponding zwitterionic, colored merocyanine (MC) form
tion, virtually all previous studies highlight regenerability of the via heterolytic cleavage of the spiro carbon–oxygen bond upon
1
1
designed adsorbents, by which the adsorption capacity of the UV irradiation, by metal ions or by polar solvent stimulus.
exhausted adsorbents can be restored and any valuable chromium Here, it is noted that a phenolate group is generated once the SP
retained can be recovered. To date, regenerability has been an photoisomerizes to the MC species. As long as the phenolate
essential criterion to evaluate the performance and, in particular, in MC remains deprotonated, the negatively charged phenoxy
cost effectiveness of a chromium-targeting adsorbent for oxygen can operate as a ligand to interact with electronically
up-scaling its application. However, to the best of our knowledge, poor species such as chromium cations. More importantly, since
the adsorbents reported previously unexceptionally require strong the MC - SP ring-closing reaction is reversible under visible
acids, alkalis, or chelating agents to sequester the chromium light, we envision that irradiation with visible light may liberate
4
,6,8
ions.
Indeed, discharging effluents containing chemical the chromium cations back into the aqueous media, leading to
desorbents themselves contribute to secondary contamination, regeneration of the adsorbents in a photo-controllable manner.
The feasibility of this strategy has been demonstrated preliminarily
in the present study by using a spiropyran derivative covalently
immobilized onto two representative solid surfaces, i.e., silicon
a
Key Laboratory of Leather Chemistry and Engineering of Ministry of Education,
Sichuan University, Chengdu 610065, P. R. China
wafer (SW) and magnetite particles (Fe O ). Since light is a
b
3 4
State Key Laboratory of Polymer Materials Engineering, Sichuan University,
Chengdu, 610065, P. R. China. E-mail: fanhaojun@scu.edu.cn
noninvasive and environmentally benign stimulus with high
spatiotemporal solution, the rationale underlying the present
effort may provide guidance for designing new adsorbents that
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8cc08099b
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address the aforementioned contamination associated with the Benesi–Hildebrand equation (Fig. 1(d)), the complex-ratio and
use of chemical desorbents, a universal problem that has not the binding constant k of the MC–Cr(III) complex were calculated
3
À1
been solved yet in remediating chromium-containing water to be 0.45 and 5.3 Â 10 M , respectively, indicating that MC
by adsorption. coordinates mildly with Cr(III) in a 1 : 1 stoichiometry (see the
To covalently immobilize spiropyran onto SW and Fe O , a ESI†). Such an SP - MC transition was reversible, with ca. 88%
3
4
spiropyran with a propionic acid side chain appended to the (ESI,† Fig. S7) of the spiropyran molecules existing in the closed
indolic nitrogen was synthesized (ESI,† Scheme S1) according SP form after irradiation with visible light (l 4 400 nm) for
1
2
to a previously disclosed procedure. The chemical structure of 20 min (ESI,† Fig. S8). Accordingly, the UV-Vis absorption spectra
the product was verified by Fourier transform infrared (FTIR) of the mixture shifted back to the original state, with only one
1
spectroscopy, H nuclear magnetic resonance (NMR) spectro- absorption detected in the visible region (ESI,† Fig. S8). This
scopy, and electrospray ionization mass spectrometry (ESI-MS) result indicated that the Cr(III) species were repelled from the
(
ESI,† Fig. S1–S6). The appended propionic acid acted as a spiropyran derivative as the MC photoisomerized to SP.
reactive point for covalent immobilization of the spiropyran Then, the spiropyran derivate was covalently immobilized onto
derivative onto solid surfaces. In addition to the phenolate two representative solid surfaces, i.e., amine-functionalized planar
produced by the heterolytic cleavage of the spiro carbon–oxygen silicon wafer (SW) and spherical magnetite particles (Fe ) by
bond, there were no other ligands (e.g., the hydroxyl group in EDC/NHS-mediated coupling, yielding two solid adsorbents
the vicinity of the phenolate group) in the spiropyran derivative; SP@SW and SP@Fe , respectively (Scheme 1). The successful
otherwise extra ligands bond irreversibly with trivalent chro- immobilization of SP onto SW and the Fe surface was con-
3 4
O
3 4
O
3 4
O
mium that could not be photochemically repelled from the firmed using various technologies. As for the synthesized SP@SW,
adsorbents. Then, a DMF (dimethylformamide)/H O solution compared with freshly cleaned SW (ESI,† Fig. S9), it was found
2
(
2
2
50 : 50 v/v) of the spiropyran derivative at a concentration of that many small mountain-like protuberances were present
À4
.5 Â 10
M was prepared. After stored in the dark for on the SW surface, displaying a mean surface coating
40 min, 38% of the spiropyran derivative isomerized into the of 100 nm (Fig. 2(a)). Especially, the successful introduction
ring-opened MC form (ESI,† Fig. S7). Accordingly, the colorless of SP moieties on the SW outmost layer was systematically
solution turned red, accompanied by the appearance of an corroborated by (1) the ATR-FTIR spectrum (Fig. 2(b)) that
À1
intense absorption band in the visible region with the peak amplified the 1450–1668 cm region and made deconvolution
À1
maximum at 530 nm (Fig. 1(a)). Upon the addition of Cr(III) of the interest displaying amide I (peak at 1654 cm ), amide II
À4
À1
(
4
2.5 Â 10 M) into the MC solution, a new absorption peak at (peaks at 1554 and 1510 cm , respectively) and Ar–NO (peak
2
À1
20 nm appeared while the absorption at 530 nm experienced at 1330 cm ) absorption bands, corresponding to the coupling
a red shift as a function of time (Fig. 1(b)), indicating amide bonds and 4-nitrophenyl from the chromene moiety,
1
3–15
the formation of an MC–Cr(III) complex.
Furthermore, respectively; (2) the high-resolution XPS spectral envelopes
) C 1s, (d ) N 1s and (d ) O 1s, simultaneously
based on Job’s method of continuous variation (Fig. 1(c)) and (Fig. 2(d)) of (d
1
2
3
detecting the SP-derived CQO, C–O, N–O, and CQC signals;
and (3) fluorescence microscopy imaging (Fig. 2(c)) demonstrating
a red glow on the SW surface, attributing to the SP moieties that
translate to a red color MC form under UV irradiation (ESI,†
Fig. S10). In addition, UV-Vis absorbance analysis of the super-
natant obtained from a residuary reaction mixture of the amide
couplings showed that B58.5 wt% of the added SP were attached
onto the SW surface. The quantity of SP moieties immobilized onto
À2
the surface of SW was therefore calculated to be 72 mmol m
Also, the successful synthesis of SP@Fe
.
3
O
4
was characterized using
SEM, TEM, XPS, and UV-Vis absorbance technologies (ESI,†
Fig. S11 and S12), and the quantity of SP moieties immobilized
À1
onto the engineered Fe O surface was 0.21 mg mg
.
3
4
Light-induced translocation of trivalent chromium (Cr(III))
around SP@SW and SP@Fe was then examined by exposing
3 4
O
3
one piece (10 Â 10 Â 0.6 mm ) of SP@SW or 150 mg of
SP@Fe
3
O
4
to 20 mL of Cr(III) solution with an initial concen-
À1
tration of 20 mg L (see the ESI,† Fig. S13) in a thermostatic
shaker. The mixture was agitated at 20 1C in the dark with a
Fig. 1 UV-Vis absorbance spectra of (a) a spiropyran derivative (2.5 Â
flow of ultrapure N for 4 h. After that, the exhausted adsor-
bents were withdrawn, washed repeatedly with water, and then
exposed to 10 mL of deoxygenized water under visible light
2
À4
1
0
2
M) in DMF/H O solution (50 : 50 v/v, 293 K) and (b) after the addition
of 1 equivalent of trivalent chromium under dark conditions; the inset
shows their absorbance and solution color change over time. (c) Job’s plot
À4
(4400 nm) with vigorous stirring for 30 min (see the ESI,†
(
(
total concentration of MC (SP) and trivalent chromium: 4 Â 10 M) and
d) Benesi–Hildebrand plot of MC with trivalent chromium.
Fig. S14) for desorbing Cr(III). Evidence supporting that Cr(III)
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Scheme 1 Synthesis and structure of the representative solid adsorbents, SP@Fe O and SP@SW.
results are presented in Fig. 3(a). The adsorption capacity of
À2
the two adsorbents for Cr(III) was measured to be 0.88 g m of
À1
SP@SW and 1.54 mg g
of SP@Fe
3
O
4
, respectively. After
with visible
light, there were still adsorbed-Cr(III) remaining on their surface
Fig. 3(a)), coinciding well with the SEM-EDS Cr-mapping and
Cr 2p XPS results. The residual Cr(III) on the solid adsorbent
3 4
irradiation of the exhausted SP@SW and SP@Fe O
(
À1
À2
surface (0.49 mg g for SP@Fe O , while 0.61 g m for SP@SW;
3
4
Fig. 3(a)) may be ascribed to the physical interaction, which
cannot be desorbed by visible light irradiation or chemical
desorbents. Furthermore, in order to evaluate the binding
capacity of the present designed adsorbents to other metal ions
n+
n+
+
+
2+
2+
(
M ), five M with different valences, i.e., K , Na , Pb , Ca ,
Fig. 2 (a
indicated by the straight line), (a
spectrum, (c) fluorescence microscopy image, and (d
) O 1s high-resolution XPS spectral envelopes of SP@SW. The decon- relative amount of M released from SP@Fe O composites by
1
) AFM height image (cross-sections were taken at the place
) SEM cross-section image, (b) ATR-FTIR
) C 1s, (d ) N 1s and
3+
and Fe , were also employed (ESI,† Fig. S15 and S16). Taking
SP@Fe for the target adsorbent, the results showed that the
2
3 4
O
1
2
3
+
(d
3
3
4
volved ATR-FTIR spectra were fitted with Gaussian band shapes via an visible light irradiation was the largest compared with those
iterative curve fitting procedure. To separate different species of the same
element, a CasaXPS processing software was employed to deconvolve
the XPS signals with a Gaussian–Lorentzian function (Gaussian = 80%,
Lorentzian = 20%) after subtraction of a Shirley background.
+
2+
of M and M (ESI,† Fig. S16), indicating that efficient light-
driven composite-regeneration was achieved with moderately
stable complexes (M [MC-moiety]), rather than with unstable
3
+
+
2+
(
M [MC-moiety]) or very stable complexes (M [MC-moiety]).
Such results further confirmed that present adsorbents
was successfully adsorbed under dark conditions was provided with a surface-bond spiropyran moiety were suitable to treat
by studies of SEM-EDS Cr-mapping and XPS of the exhausted M -contaminated water, exemplified by Cr , realizing the
SP@SW and SP@Fe O . As illustrated in Fig. 3 (b and b , after regeneration of the adsorbents in a photo-controllable manner.
3
+
3+
3
4
1
2
treatment under dark conditions), the Cr element distributed
evenly on the surface of the exhausted adsorbents. Accordingly, was carried out. The gradual decay of the adsorption capability for
the high-resolution Cr 2p XPS spectrum (Fig. 3; c and c , after both adsorbents could be ascribed to photodegradation of those
treatment under dark conditions) revealed two signals, with BE immobilized SP. In particular, B27% (SP@SW) and B14%
peaks at 576.5/587 eV for SP@SW and 577.2/586.9 eV for (SP@Fe ) of immobilized SP photo-degraded after three switching
In compar- cycles (ESI,† Fig. S17), calculated from the change in adsorption
ison with the control (Fig. 2(d ) and ESI,† Fig. S12(d )), the capability of chromium. However, it seemed like photodegradation
Subsequently, repeated regeneration of the exhausted adsorbents
1
2
3 4
O
1
6,17
3 4
SP@Fe O , corresponding to the Cr(III) element.
2
2
deconvolution of the N 1s spectral envelop (Fig. 3; c and c , did not occur for those free SP counterparts in DMF/H O solution
1
2
2
after treatment under dark conditions) revealed an extra com- under the same conditions (ESI,† Fig. S18). As documented
1
1,18,19
ponent with BE peaks at 400.9 eV for SP@SW and 400.2 eV previously,
immobilized SP has numerous advantages over
+
16
for SP@Fe O , ascribed to N from the indoline moiety. In the free counterpart including reduced photodegradation. The
the case of the regenerated adsorbents, the N component present results are inconsistent with the prediction, and a
disappeared in the corresponding N 1s spectral envelop (Fig. 3; precise reason for this observation is still open to debate.
3 4
+
c
1
2
and c , after Vis treatment), indicating that the MC moieties
In conclusion, we demonstrate that light-induced, reversible
were transformed into the SP form. However, the Cr signal still isomerization of surface-bound spiropyran can be harnessed to
appeared in the SEM-EDS Cr-mapping (Fig. 3; b and b , after Vis desorb trivalent chromium. Since visible light is a noninvasive
1
2
treatment) and Cr 2p XPS spectra (Fig. 3; c
1
2
and c , after Vis and environmentally benign stimulus with high spatiotemporal
treatment). Furthermore, to quantify the photo-mediated Cr(III) solution, the rationale underlying the present effort may
translocation around the adsorbents, the Cr(III) concentration in provide guidance for designing new adsorbents that address
the residual solution was determined via ICP-OES, and the secondary contamination derived from chemical desorbents,
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Fig. 3 (a) Adsorption (Ad.) amount of Cr, (b) zero-loss SEM image and the corresponding EDS Cr-mapping, and (c) Cr 2p and N 1s high-resolution
XPS spectra of SP@SW (a , b and c ) and SP@Fe (a , b and c ) during the first recycling under the dark and irradiation by visible light (Vis), respectively.
d) Mechanism proposed for Cr(III) reversible photocontrollable coordination–dissociation by the solid adsorbents. A CasaXPS processing software
1
1
1
3
O
4
2
2
2
(
was employed to deconvolve the XPS signals with a Gaussian–Lorentzian function (Gaussian = 80%, Lorentzian = 20%) after subtraction of a
Shirley background.
bringing adsorption techniques one step closer to sustainability
in treating chromium-contaminated water.
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The authors gratefully acknowledge financial support of
this work from the National Key Research and Development
Program of China (2017YFB0308600), the National Natural
Science Foundation of China (21576172), and the Fundamental
Research Funds for the Central Universities (2012017yjsy177).
We also thank Zhonghui Wang (College of Light Industry, Textile
and Food Engineering, Sichuan University) for her help in con-
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Conflicts of interest
2
The authors declare no competing financial interests.
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